Computer readable marks on printed pages are commonplace, with many varieties, such as the common barcode, in extremely widespread use. Although the common barcode is ubiquitous, an increasing number of alternative computer readable marks are reaching the marketplace. Many of these new varieties of marks provide much greater data carrying capacity than the common barcode, enabling a wider range of applications than is possible with the limiting 30 to 60 bits of data that can be stored in a common barcode. Other varieties of marks improve on the common barcode by exhibiting reduced visibility, which has the advantage that a larger portion of the page is left free to contain human-readable content. An additional advantage of low visibility marks is that information can be hidden in a page, enabling applications such as siteganography and watermarking.
As wider use is made of computer readable marks, and new applications for computer readable marks are found, there is an increasing need for marks that provide a high data content without consuming large areas of the page. Such marks leave most of the page free to contain human-readable content, which is desirable for many applications.
When recovering watermarks or other steganographs from images, the problem of determining the correct orientation for reading the mark arises, because the marks are by definition obscured from sight, and hence will often be scanned or otherwise digitized with a random orientation.
Many methods for determining the correct orientation of steganographs have been disclosed, such as incorporating and later detecting multiple one dimensional orientation signals into the image. Even if these signals are only weakly embedded, to the point where the signals may be imperceptible to the human eye, an appropriately oriented projection of the image will reveal the presence of a one dimensional carrier, as described in Australian Patent Application Serial No. 2002951815 (Canon Kabushiki Kaisha).
Australian Patent Application No. 2005209709 (Canon Kabushiki Kaisha) describes another form of steganograph that uses a fixed header message in a predetermined location to aid in the determination of orientation.
The addition of orientation signals to steganographs is a well known technique that allows for the ready determination of orientation at the expense of increased visibility. When producing steganographs where visibility must be minimised, the presence of such orientation signals is undesirable.
Several methods for preventing unauthorized copying of image data exist. These methods are usually based on locating fixed patterns or indicia in an image that is to be copied. Alternatively, some methods have data encoded with reference to a virtual grid. A need exists to remove the need for having a fixed distance grid, and provide a faster methodology to detect a reference grid and separate the reference grid from data carrying dots. A further need exists to provide a faster methodology to detect a reference grid by selectively evaluating only the marks forming grid-like reference patterns while ignoring data-carrying marks.